libata: fix probe time irq printouts

Most drivers don't seem to fill out the host->irq field, resulting in the
wrong (no) irq being reported at probe time. For example, sil24 on my system:

ata1: SATA max UDMA/100 cmd 0xd00008009001f000 ctl 0x0000000000000000 bmdma 0x0000000000000000 irq 0
ata2: SATA max UDMA/100 cmd 0xd000080090021000 ctl 0x0000000000000000 bmdma 0x0000000000000000 irq 0

Since they're allocated and set up in ata_host_activate(), just save
them away there.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>